This invention relates to a novel sphere making apparatus and more particularly to such an apparatus for making spheres from minute particles of glass or other sphere forming material and to the spheres produced thereby.
Glass beads and other spherical particles manufactured in accordance with the invention have numerous industrial and commercial applications. In many cases the beads are used to provide a reflecting surface, such as in lane marking for highways, for road and advertising signs, motion picture screens, etc. Other uses for the beads include applications in which their reflecting properties are of little moment, as in cases in which the beads are employed as fillers for plastic materials, for impact treatment and peening of metal surfaces, or for various electrical uses. The diameter of the beads may vary widely and illustratively ranges from about 6.0 millimeters down to about 1 micron.
Various processes and apparatus have been employed heretofore to manufacture glass spheres. As an illustration, it often has been common practice to introduce irregularly shaped glass particles into a vertically disposed draft tube which is open at its upper end and is provided with a well-distributed gas flame adjacent its lower end. The particles are carried upwardly by the combustion gases into an expansion chamber or stack mounted above the draft tube. During their upward movement, the particles become soft and are shaped by surface tension into a substantially spherical configuration to form glass beads. For a more detailed discussion of representative bead manufacturing systems of this type, reference may be had, for example, to U.S. Pat. No. 2,334,578 granted Nov. 16, 1943, to Rudolf H. Potters, U.S. Pat. No. 2,619,776 granted Dec. 2, 1952 to Rudolf H. Potters, U.S. Pat. No. 2,945,326 granted July 19, 1960 to Thomas K. Wood and to U.S. Pat. Nos. 3,560,185 and 3,560,186 granted Feb. 1, 1971 to Arthur G. Nylander.
In other cases glass spheres have been produced directly from a stream of molten glass as shown, for example, in U.S. Pat. No. 3,279,905 granted Oct. 18, 1966 to Thomas K. Wood et al. Still further sphere making processes of the type previously employed include the use of rotary kiln. In these latter processes the crushed glass particles customarily are coated with a resin or other binder and a material such as graphite to provide a protective coating and/or matrix around each particle as the spheres are formed. Processes of this latter type are disclosed in U.S. Pat. No. 3,597,177 issued Aug. 3, 1971 to Charles Davidoff and U.S. Pat. No. 2,461,011 issued Feb. 8, 1949 to N. W. Taylor et al.
The prior processes and apparatus employed in the manufacture of spherical particles such as glass beads have exhibited certain disadvantages. As an illustration, the overall thermal efficiency of many such prior systems was comparatively low, with the result that the manufacturing cost of the beads was excessive. In addition, and this has been of special moment in processes and apparatus which used a vertical draft tube, the thermal efficiency was further impaired because of the need to use a portion of the available energy for the vertical transport of the particles, and the temperature gradient within the tube resulted in the production of spheres which occasionally exhibited a lack of roundness and had other defects. It was also necessary to carefully control the population density of the particles in order to minimize the incidence of collisions between particles which detracted from the quality of the product. The equipment previously employed to produce glass spheres was large in size and had additional disadvantages which further detracted from the efficient and economical manufacture of the spheres on a continuous large volume basis.
Other prior processes and apparatus, such as those utilizing rotary kilns and similar equipment, had the disadvantage that the coating materials employed required either a binder for the protective coating or a matrix of substantial mass that needed to be heated in addition to the particles. A further disadvantage of processes and apparatus of this latter type was the fact that the coating material had to be removed in a costly mechanical process like washing, etc. to achieve a coating free product.